In src/common/compress_zstd.c, we use the `DISABLE_GCC_WARNING()` macro to disable the `-Wunused-const-variable` warning. Some versions of gcc don't recognize that warning option, so they will warn about incorrect pragma usage. ``` src/common/compress_zstd.c:29:1: warning: unknown option after ‘#pragma GCC diagnostic’ kind [-Wpragmas] DISABLE_GCC_WARNING(unused-const-variable) ^ ``` We should probably define a macro if `-Wunused-const-variable` is supported by gcc and update the preprocessor conditional in compress_zstd.c ``` #ifdef HAVE_ZSTD DISABLE_GCC_WARNING(unused-const-variable) ``` to use it. Reported on IRC. Confirmed on Xenial with gcc 5.4.0-6ubuntu1~16.04.10 and libzstd1-dev 1.3.1+dfsg-1~ubuntu0.16.04.1. Travis is still on Trusty so doesn't have a libzstd-dev. This might be a regression on platforms with libzstd but an older gcc.